Add external ID field to contacts

[wmnnd]

Contacts should have an external_id field to allow synchronization with outside databases.

• [ ] The external_id field should allow for 36 characters (string representation of a UUID) to allow for all reasonable use-cases.
• [ ] - [ ] external_id needs to have a unique constraint ([:external_id, :project_id])
• [ ] The external_id field should be used for CSV imports to detect existing entries
• [ ] There should be a way to use the contacts API with external IDs instead of Keila IDs